ISFET structure

The measurement of changes of the surface potential Vo at the interface between an insulator and a solution is made possible by incorporating a thin film of that insulator in an electrolyte/insulator/silicon (EIS) structure. The surface potential of the silicon can be determined either by measuring the capacitance of the structure, or by fabricating a field effect transistor to measure the lateral current flow. In the latter case, the device is called an ion-sensitive field effect transistor (ISFET). Figure 1 shows a schematic representation of an ISFET structure. The first authors to suggest the application of ISFETs or EIS capacitors as a measurement tool to determine the surface potential of insulators were Schenck (15) and Cichos and Geidel (16). 

Figure 1. Schematic representation of the basic pH-sensitive ISFET structure, assuming that the insulator is SiOj.

Figure 1. Schematic drawings of a MOSFET and an ISFET structure...

Figure 5. SEM photograph of a part of a wafer showing ISFET structures with a 25 im thick polyHEMA membranes and a 80 pm thick double membrane (pHEMA and polysiloxane). (Reproduced with permission from ref. 15. Copyright 1991 Sens, and Actuators.)...

Fig. 14. (a) Schematic ISFET structure without reference. Source and drain are protected from solution the oxide is hydrated, (b) Schematic of an ISFET structure with reference electrode. Source and drain are protected, (c) Schematic of a Pd gate CHEMFET. Source and drain are protected. 

As basic sensor a pH-l FET chip was developed which can be used for different ISM depositions. It contains a dual ISFET structure (n-channel FET, LP-CVD Si3N4 gate membrane, gate size 16 x 400 pm) and a temperature sensitive diode. The chip is fabricated in a standard MOS>production line. For using the chip in a ISFET difference measuring mode without a conventional reference electrode an integrated pseudo reference electrode has been designed too. 

The first dielectric material that was used on open gates of field effect transistors (ion-sensitive field effect transistor, ISFET) was silicon dioxide by Bergveld in 1970. The problem in the ISFET structure is the poor insulation between the device and the solutions. The ISFET structure was improved by using ion-sensitive electrodes (ISE). With field effect transistors the gate area can be extended by using a conductive wire covered with the sensitive membrane. This new approach helps in enhancing the stability, sensitivity, and flexibility in shaping i.e., miniaturization of pH-sensitive devices. 

MJ. Schoning, M.H. Abouzar, S. Ingebrandt, J. Platen, A. Offenhauser, and A. Poghossian, Towards label-free detection of charged macromolecules using field-effect-based structures scaling down from capacitive EIS sensor over ISFET to nano-scale devices, in Mater. Res. Soc. Symp. Proc. 915, 0915-R05-04 (2006). 

Relevant for our discussion is the genesis of the sensitivity behaviour in a class of devices all generated from the well known MOSFET structure (ISFET and GASFET). In particular, the influence of charges into the gate oxide on the threshold voltage and MOSFET behaviour under shrinking conditions will be discussed. 

Biological Membranes Attractive supporting membranes for commercial exploitation of biological material, e.g. immobilization ot membrane bound enzymes in solid state sensors ISFET tvpe structures. 

J.P. Kloock, L. Moreno, A. Bratov, S. Huachupoma, J. Xu, T. Wagner, T. Yoshinobu, Y. Ermolenko, Y.G. Vlasov and M.J. Schoning, PLD-pre-pared cadmium sensors based on chalcogenide glasses—ISFET, LAPS and pISE semiconductor structures, Sens. Actuators B Chem., 118 (2006) 149-155. 

Because of its particular technological characteristics, the discussion of the ICD shall remain confined to those made of c-Si therefore it will not be considered in this section. On the other hand, only those structures having a configuration adaptable to a-Si H thin-film technology will be taken into consideration. Two kinds of FETs are referred to in the literature the ion-selective FET (ISFET) and the gas-sensitive FET (CHEMFET). 

Hydrogenated amorphous silicon FET structures are very important candidates for ACSs especially in the field of ISFETs. The long experience with c-Si FETs in the past years (involving membranes, electrode theory, and integrated readout systems) can be advantageously used. 

The ISFET is deduced from the MOSFET, which is a well-known electronic device. Cross section of the MOSFET and the ISFET are shown in Fig. I. A MOSFET consists of a p-type silicon substrate in which two n-type diffusions are realized, which are called the source and the drain. The structure is covered with an insulating layer (usually Si02), and a metal gate electrode is deposited over the area between the source and the drain. 

Fig. 1. (a) Structure of eui ion-sensitive FET transducer with single FET element, (b) Structure of an ion-sensitive FET with metal oxide semiconductor FET. D, drain S, source IG, ion-sensitive gate CP, contact pad G, gate RE, reference electrode AS, analyte solution MOSFET, metal oxide semiconductor FET ISFET, ion-sensitive FET. 

Fig. 11. Schematic diagram of continuous flow apparatus and structure of an enzymatically coupled FET. (a) Schematic diagram of continuous flow apparams S, enzymatically coupled FET sensor SC, sensor cell WB, water bath D, drtdnage P, peristaltic pump TV, three-way joint EV, electrical valve VC, valve controller WS, washing solution AS, analyte solution, (b) Detailed structure of flow-through cell OR, rubber O-ring. (c) Structure of enzymatically coupled FET (electrical insulation with epoxy resin is not shown here for simplicity) ISFET, ion-sensitive FET EM, enzyme membrane G, thin gold film TC, card edge connector. (Reproduced from Shiono et al. (9), with permission.)...

Figure 26. Structure of an ISFET. The Mn02 nanoparticles and LOD are layer-by-layer self-assembled on top of the sensitive membrane. Successive response of the three-multilayer film based ENFETwith (a) and without (b) Mn02 nanoparticles to lactate in 10 mM PBS (pH 7.4). Inset calibration curve of theENFET with (a) and without (b) Mn02 nanoparticles to lactate (J.J. Xu, W. Zhao, X. L. Luo, H. Y. Chen, Chem. Commun. 2005, 792-794 Reproduced with permission of The Royal Society of Chemistry).

Design and fabrication of ISFET was described in Ref. [88] The interest in ISFET arises chiefly from their application as pH and ion sensors. A graphical procedure to find PZC from capacitance-voltage characteristics of electrolyte-insulator-semiconductor and metal-insulator-semiconductor structures was discussed [89]. Due to the choice of electrolyte (2 mol dm Na2S04) the PZC values reported in this study (2.5 for Si02, 2.8 for Ta20s and 3-3.4 for Si3N4) are not likely to be the pristine values due to specific adsorption of anions. 

Fig. 10.1 (a) Basic design of ISFET. (b) characteristics of the device, (c) pH responsibility of the device, (d) Change in the surface structure and surface potential of silicon nitride layer in each pH solution... 

Since the ISFET is based on the field-effect transistor, let us recall briefly how the latter operates (see, e.g.. Ref. 98). The field-effect transistor (Fig. 19a) represents the so-called MIS (metal-insulator-semiconductor) structure (hence the abbreviation MIS-FET), i.e., a semiconductor base, onto which an insulating layer and a metal electrode (gate) are deposited. The base usually is a p-type silicon plate and the insulator, a Si02 or Si3N4 layer. With a thickness of 100-200 nm, the resistance of this layer is of the order of 10 fl. Two regions are produced in the base by local... 

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